1. Field of the Invention
This invention relates to a parallel connected power supply consisting of at least two independent supplies implemented using single wire current sharing and, in particular, to the connection of at least two power supplies in parallel in such a manner as to provide redundancy or higher output capability for the parallel-connected supplies.
2. Prior Art
As described in an article by Reeves and Mehta, published in Electronic Products, Sept. 7, 1982, on pages 106 to 108, and entitled, "Minimizing the Effects of Power Supply Failure", a number of ways exist for achieving redundancy in power supplies. A redundant power supply consists of at least two independent supplies, each of which can carry the entire load, connected in parallel. If one supply fails, the other maintains the system's operation. As described by Reeves and Mehta, a number of different approaches are commonly used to achieve redundancy. One common approach involves the use of a "master-slave" system, in which a master supply controls the operation of one or more slave systems. While each supply delivers an equal share of the total load current, should a master fail then the entire system fails defeating the purpose of a redundant supply. However, because each supply is operating well below its rated capacity, the probability of failure of a particular supply is less than when the supply is operated at its maximum capacity.
A second approach uses an external load sharing or current balancing module. Output cables from each supply are connected to the module and a single pair of cables connects the module to the supply bus. Control circuitry within the module feeds signals back to the supplies commanding them to share load current equally. Each supply achieves a longer life because it is operated beneath its maximum output. However, the control module must be built to accommodate the number of parallel supplies. Should additional supplies be added to the system in the field the control module must be replaced.
A third approach consists of the direct paralleling of two or more supplies. If the load demands more current from a supply than a preadjusted fold back limit, internal circuitry automatically reduces or folds back the output current of one of the supplies to a percentage of its rated output. At this point the second supply begins to deliver power. Using this technique, any number of supplies can be connected together. No external control circuitry or module is required. However, current is not shared equally among the supplies so one or more supplies are under greater stress and thus are more likely to fail than the other supplies.
A fourth technique is known as "single wire current sharing". This technique provides the benefits of direct paralleling plus an equal distribution of current among the parallel supplies. In the single wire system, each supply contains special load current sensing and control circuitry and transmits information about output current to other supplies in the system via a single wire communications loop. The control circuitry in each supply adjusts the output current to equal that being supplied by the other units in the system. As a result, load current is shared equally by all the supplies. The major disadvantage to the single wire approach is that the supplies used must have the required special internal control circuitry.
Thus in summary, the use of a number of power supplies in parallel to increase reliability and provide more total output current is common. If extra current capability is provided, one or more supplies can fail without causing an output voltage failure. The good supplies "pick up" the extra load. The paralleling of supplies increases reliability by decreasing the operation of the supplies at their maximum rated levels. By paralleling supplies in a manner to insure equal load sharing, each supply operates at a reduced load. This increases the reliability of each supply by lowering its operating temperature and increasing its life. All paralleling methods except simple direct paralleling provide equally shared load current. The failure of simple direct paralleling to do so makes this technique unacceptable for most applications. The disadvantage of a master-slave paralleling system is that if the master fails the entire system shuts down. In addition, the master is usually different from the slaves and cannot always be interchanged with the slaves. This makes the system more complex than a system utilizing simple, interchangable supplies with no external circuitry. Another problem with parallel wire redundancy circuits is that the interconnecting paralleling wire might pick up external noise. Unless the paralleling circuit filters this noise, it will not be immune to this noise. This precludes many possible circuits lacking the appropriate filters from being practical. In addition, short circuiting of the parallel bus will disable the system and prior art systems also exhibit stability problems. The direct coupling of "error" signals from several supplies is similarly impractical.
In view of the above, the single wire direct paralleling approach is preferred.